1. Field of the Invention
The present invention relates to a control apparatus and a control method for stabilizing the operation of an electro-optic modulator included in an optical transmitter of an optical communication system or an optical transmission system, and more particularly, to an apparatus and a method for enabling an electro-optic modulator to maintain the type and the strength of an optical output signal even if a transfer function in the electro-optic modulator changes, thereby stabilizing the output of the electro-optic modulator.
2. Description of the Related Art
Generally, an electro-optic modulator includes two interferometric waveguides, and indicates periodical optical output characteristics by delaying phases of the waveguides. In the electro-optic modulator, a Radio Frequency (RF) electrode is positioned between two waveguides in order to induce the phase delay. In the electro-optic modulator, there is also a bias electrode designed to adjust a bias voltage. Inappropriate bias voltage of the electro-optic modulator may cause an optical output signal of the electro-optic modulator to be distorted.
An optical input signal, which carries an electrical dithering signal that has a frequency different from that of an electrical information signal, is applied to an existing apparatus for stabilizing optical outputs of the electro-optic modulator. To be more specific about the existing stabilization method, the optical output stabilizer extracts one of a primary wave, a secondary harmonic or an Nth harmonic of the dithering signal. Then, the stabilizer detects an error signal corresponding to the variance of the frequency phase of the dithering signal, and stabilizes the output of the electro-optic modulator by controlling a bias voltage through the feedback control of the error signal.
However, a band filter is necessary to extract one harmonic from an optical detector, and due to noises after the band filtering, it is difficult to maintain an accurate operation point. If the dithering signal with higher amplitude is provided to the electro-optic modulator in order to reduce the impact of noises, the dithering signal may work as noises to the electrical signal indicative of information. In addition, it is hard to adapt to the change of the 180xc2x0 phase modulation voltage.
To solve the above-described problems, it is an object of the present invention to provide an apparatus and a method for controlling an electro-optic modulator by initializing a bias voltage and gains of an electrical information signal so that the electro-optic modulator can operate in optimized conditions, thus enabling the electro-optic modulator to maintain stable characteristics of an optical output signal even if a transfer function of the electro-optic modulator changes.
It is another object of the present invention to provide an apparatus and a method for controlling an electro-optic modulator by adaptively adjusting a bias voltage applied to the electro-optic modulator in an optical transmitter so that the electro-optic modulator can maintain an optical output stable.